Multi-select in file dialog

It's not immediately apparent that the open dialog will go away the moment you tap a file, especially with an "Open" button sitting right there.

Better idea would be to let the user select multiple files and then press the Open button.

Start by locating the open-dialog entry point and the existing Open button handling. Make the dialog keep its selections so multiple files can be chosen before confirming with Open; done means tapping a file does not immediately close the dialog and the selected files are opened together.

Written by the indexing model from the issue text.